1. Field of the Invention
The process which is the subject-matter of the invention relates to the detection of defects using eddy-currents in metallic tubes cold-rolled using a pilger mill.
2. Discussion of the Background
A process is known for detection using eddy-currents which is applicable in particular to the testing of small diameter tubes. Such tubes are used more especially in heat exchangers for the production of steam using pressurised water passing through these tubes in a closed circuit from a nuclear reactor. These tubes are most often made with certain stainless steels such as a type 316 steel (ASTM A312 standard) or INCO 600 and 690 or otherwise other alloys with a high Ni content, such as INCOLOY 800 and INCOLOY 825 (ASTM B163 standard), INCO and INCOLOY being trademarks of HUNTINGTON ALLOYS.
The process for detection of defects using eddy-currents is used in a systematic manner for 100% testing for the defects which these tubes may present either at the time when they are supplied or subsequently in tests carried out directly on the tubes assembled on the exchangers when there are stoppages, usually programmed, for checking operating performance.
The conditions for carrying out such eddy-current tests are the subject of regulations proposed, for example, by ASME and applied most often by agreement between the producers and users, in order to ensure the most efficient detection possible of the defects presented by the tubes. These tests can be carried out either with external sensors or with internal sensors. Standard defects are used, in general composed of small diameter holes radially piercing the wall of these tubes through the whole thickness or a fraction thereof, for comparing eddy-current responses thereto with responses received in examining a tube with unknown defects.
For eddy-current testing of the in-service inspection type, the tubes being assembled in the exchangers, detection is carried out from the interior of the tube by means of an internal sensor with a diameter slightly less than the diameter of said tube, which is displaced at a relative speed of, for example, 0.30 m/s.
The sensor is generally composed of 2 coils mounted in a Wheatstone bridge and used in differential mode. The sensor is supplied with high frequency current in a typical range of 1 00 to 700 KHz.
By way of example, the exchanger tubes used for the production of steam from pressurised water most often have an external diameter of approximately 0.75 to 1.00 inch (19.05 to 25.4 ram) and a wall thickness of approximately 0.026 to 0.043 inches, that is to say approximately 0.66 to 1.1 mm.
It has been noted that the significant reduction in section, generally greater than 50% and which can exceed 70%, carried out cold in one pass using a pilger mill leads to a very significant work-hardening which gives the advantage of avoiding an excessive coarsening of the grain of the alloy, during solid solution annealing which is most often carried out to limit the risks of stress corrosion.
It has also been noted that cold-rolling carried out in this manner is the cause of a relatively large background noise which appears at the terminals of the eddy-current sensor receiving means. The background noise reduces the signal/background noise ratio and thus tends to mask low amplitude defects.
It is possible to lower the background noise by modifying the rolling conditions but it is most often necessary to reduce the rolling speed, and therefore productivity; despite this, the reduction in the background noise is often insufficient to achieve the signal/background noise ratio required for a standard defect of set dimensions.
The possibility has been looked into of reducing, in a significant manner, the background noise observed during eddy-current testing in an internal or external sensor of tubes cold-worked using a pilger mill. It has also been sought not to have to reduce the rolling speed, and also not to have to lessen the amount of reduction and thus of the work-hardening before the final heat solution treatment.
Finally, the possibility has been looked into of perfecting a process for section reduction before final annealing, generally applicable to the production of all types of small diameter, long length metallic tubes allowing eddy-current testing for defects by external or internal sensor with a high signal/background noise ratio.